Photosynthesis

Word and symbol equations

carbon dioxide + water → glucose + oxygen   (using light energy)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Key facts

• Endothermic – it absorbs energy (from light).
• Happens in chloroplasts, which contain chlorophyll.
• Glucose is used to: respire, store as starch, make cellulose (cell walls), make proteins (with nitrate), and store as oils/fats.

Limiting factors

The rate is limited by whichever is in shortest supply:

• Light intensity
• Carbon dioxide concentration
• Temperature (too high → enzymes denature)
• (chlorophyll amount)

Reading a rate graph

The line rises then plateaus when another factor becomes limiting. To get more, increase the factor that has levelled off.

Inverse square law (light)

Light intensity ∝ 1 / distance². Doubling the distance quarters the light intensity (used in the pondweed practical).

Exam tip

Always name which factor has become limiting when a graph plateaus. Photosynthesis is endothermic and stores energy; respiration releases it.